pip install jupyterhub==0.8.0
There are significant changes to internal communication between the Hub and single-user servers, so it is important when you upgrade JupyterHub to upgrade the version inside your user environments as well. This means adding a line like

pip install jupyterhub==0.8.0
to your Dockerfiles if you are using a container-based deployment to ensure that the Hub and single-user servers have the same version.